There exists a need for burners for gasses with a low calorific value. These are usually recovered gasses coming from industrial processes (blast furnace, biogas, gas from discharges, gas issuing from various gasification processes, furnace gases containing VOCs, etc.), the calorific value of which is low because of their high concentration of inert gas. This is the case with certain types of synthesis gas. “Synthesis gas” (or “syngas”) means a mixture of CO/H2 issuing from the gasification of carbonaceous substances in the presence of steam and/or air, optionally enriched with oxygen. In particular, the synthesis gas obtained from the conversion of biomass in a gasifier without oxygen enrichment has a much lower calorific value than that of fossil fuels (in particular propane and butane gases); this is a “gas with a low calorific value”.
“Lean gas” or “gas with a low calorific value” means a gas with a calorific value (NCV) of less than 3000 Kcal/m3.
Moreover, these gases of various origins, such as synthesis gas issuing from biomass, often contain tars and particles. Because of this, use thereof in the burners normally used for fossil fuels is generally not possible, and it is necessary to develop specific burners for synthesis gas (or other lean gases). Such specific burners exist, but are complex in design.
In particular, French Patent No. FR 2 889 292 describes a burner for a lean combustible gas of the type comprising a combustion nose on a central axis and means for supplying a mixture of combustible gas and combustion air in rotation about the central axis of the burner. The burner is distinguished in that it is configured so as to inject, in front of the combustion nose, a flow of non-flammable pre-mixture containing a mixture of premixed air and combustible gas, a complementary flow so as to reach a flammability threshold of the mixture in front of the combustion nose, said flow being ejected at the centre of the pre-mixture flow by means of a central complementary air flow and/or around the pre-mixture flow by means of a peripheral complementary air flow. This configuration has complexity of assembly and adjustment of the means used with the additional drawback, if the lean gas contains tars and particles, of gradual fouling of the equipment, in particular in the pre-mixture zone before ignition of the fuel.
European Patent No. EP 1 800 062 describes a burner for the combustion of a gas with a low calorific value, comprising a channel extending along an axis of the burner, for the supply of combustion air, and a channel for the combustible gas, which is designed for a large flow, in terms of volume, of combustible gas with a low calorific value, the channel for the gas and the channel for the air emerging in a mixture zone. The channel for the air has an orifice zone immediately adjacent (in flow technology) to the mixture zone, and a swirling element, for producing turbulent combustion air, is provided in the orifice zone and swirling fins are disposed in the channel for the air upstream of the swirling element. Because of its configuration, this burner does not minimise the production of nitrogen oxides, and there exist risks of pulsing of the flame. In addition, it is not suitable for gases containing tars since the tars are liable to condense on the cold wall of the pipe and the pipe introducing the lean gas is sensitive to fouling (annular zone). The mixture of air and lean gas is no doubt fairly non-uniform.
The subject matter of U.S. Pat. No. 7,003,957 is a synthesis-gas burner. The combustible gas is injected through holes inclined in the radial direction towards the centre of the combustion chamber, and having a diameter D and an injection angle alpha. These holes are placed at the discharge from the burner, namely at the end of the swirling space. The diameter D and the angle alpha are specific parameters that are chosen suitably by a person skilled in the art according to different variables (specific gas composition, emissions, etc.). This burner is very sensitive to fouling since the lean gas is injected through holes, and because of this the burner is not suitable for the presence of tars in the lean gas. It is a burner without a pilot flame, which poses a problem for ensuring combustion stability. Because of its configuration, this burner certainly produces a great deal of nitrogen oxides, and there exist risks of pulsing of the flame.
In addition, the European Patent No. EP 0 008 842 describes a burner capable of simultaneously burning several gaseous fuels with different calorific values. The burner for multiple gaseous fuels uses forced-draft preheated air, and comprises a cylindrical inner burner tube, means being provided for injecting, through the upstream end, combustion air preheated to a selected temperature and compressed to a chosen pressure. The downstream end of the inner burner tube is closed, and a plurality of longitudinal slots are spaced apart circumferentially in the wall of the tube at the closed end. Means are provided for injecting a rich combustible gas at a chosen pressure in the inner burner tube along its axis. An outer burner tube axially surrounds the inner burner tube and forms an annular passage. Means are also provided to pass a lean combustible gas at low pressure through the annular passage and outside through a peripheral slot at the end of the outer burner tube inside the furnace. This burner is very sensitive to fouling since the lean gas is injected through holes, and the burner is not suited to the presence of tars in the lean gas. It is a burner without a pilot flame, which poses problems for ensuring stability of combustion. Because of its configuration, this burner certainly produces a great deal of nitrogen oxides, and there exist risks of pulsing of the flame. Thus burner produces a flat flame, without the possibility of extending it, which destines it for specific furnaces.
European Patent No. EP 0 780 630 B1 (Alstom) describes a burner comprising a swirler followed by a tube, this tube itself being followed by the combustion chamber proper (in the direction of gas flow). The junction zone between the tube and the combustion chamber (referred to as “A” in the description of the patent) has a particular form that enables a “detachment edge” to be formed, the role of which is to stabilise a reflux zone. This reflux zone fulfils the role of a “flame attacher.” This burner is very sensitive to fouling since the lean gas is injected through holes, and the burner is not suited to the presence of tars in the lean gas. It is a burner without a pilot flame, which poses problems for ensuring combustion stability. Because of its configuration, this burner certainly produces a great deal of nitrogen oxides, and there exist risks of pulsing of the flame.
The applicant has found that the existing burners suitable for the combustion of synthesis gas are complex, difficult to adjust and to operate, and generally subject to fouling. There therefore exists a need for a lean-gas burner of simple design that is simple to manufacture and of low cost, reliable, easy to adjust, and not requiring any or only a little maintenance.